NOTICE: this is the author’s version of a work that was accepted for publication in Lithos. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Lithos, Vol.190-191, (2014)]. DOI: 10.1016/j.lithos.2013.11.015 *Manuscript Click here to download Manuscript: KunzManuscript.doc Click here to view linked References 1 Partial melting of metabasic rocks in Val Strona di Omegna, Ivrea Zone, 2 northern Italy 3 4 Barbara E. Kunz a,1,*, Tim E. Johnson a, Richard W. White a, Charlotte Redler b 5 6 a Earth System Science Research Centre, Institute for Geosciences, University of Mainz, Becherweg 21, D- 7 55099, Mainz, Germany 8 b Institute of Earth and Environmental Sciences, University of Freiburg, Albertstrasse 23b, D-79104 Freiburg, 9 Germany 10 11 *Corresponding author. Tel.: +41 (0)31-631 4738; fax: +41 (0)31 631 4843. 12 E-mail address: [email protected] (B.E. Kunz). 13 1 Present address: Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, 14 Switzerland. 15 16 ABSTRACT 17 18 Field and petrographic observations combined with major and trace element bulk rock 19 geochemistry show that metabasic rocks within Val Strona di Omegna in the central Ivrea 20 Zone partially melted during granulite facies regional metamorphism. A transition from 21 granoblastic amphibolite facies metabasic rocks at the lowest metamorphic grades to 22 metatexitic and diatexitic migmatites in the granulite facies records the effects of in situ fluid- 23 absent partial melting. Coarse-grained euhedral clinopyroxene porphyroblasts within 24 leucosomes are consistent with anatexis via incongruent fluid-absent melting reactions 25 consuming hornblende, plagioclase and quartz to form clinopyroxene and melt. Field 26 observations are supported by bulk rock geochemistry, in which high-grade samples are 27 generally depleted in mobile elements relative to unmigmatised mid amphibolite facies rocks 28 that may approximate pre-melting protolith compositions. Many of the metabasic rocks at the 29 highest-grade parts of Val Strona di Omegna probably belong to the Kinzigite Formation and 30 are unlikely to be part of the younger Mafic Complex as previously proposed. 31 32 Keywords: In situ partial melting, metabasic rocks, granulite facies regional metamorphism, 33 Ivrea Zone 34 35 36 1. Introduction 37 38 The main driving forces for differentiation of the Earth are partial melting and 39 buoyancy-driven migration of melt. These processes are irreversible and have led to the 40 pronounced physico-chemical structure of the continental crust (e.g., Brown and Rushmer, 41 2006; Sawyer et al., 2011). As direct observation of these processes is not possible, 42 information regarding pressure–temperature (P–T) conditions, melt compositions and the 43 degree and mechanisms of the production, segregation and migration of melt are largely 44 derived from the study of migmatites (e.g., Sawyer, 2008). 45 The Ivrea Zone in northern Italy (Fig. 1) exposes a section through the mid to lower 46 continental crust and has been the focus of numerous studies, most of which have 47 concentrated either on metapelitic rocks within the Kinzigite Formation (e.g., Barboza and 48 Bergantz, 2000; Bertolani, 1968; Ewing et al., 2013; Handy et al., 1999; Luvizotto and Zack, 49 2009; Mehnert, 1975; Redler et al., 2012, 2013; Zingg, 1978) or on the layered mafic 50 intrusions from the Mafic Complex (e.g., Peressini et al., 2007; Quick et al., 1992, 1994, 51 2009; Rivalenti et al., 1975, 1981; Sinigoi et al., 2011). The river section of Val Strona di 52 Omegna preserves a near-continuous metamorphic field gradient from mid-amphibolite to 53 granulite facies conditions in which metapelitic compositions preserve a transition from 54 unmelted (subsolidus) to partially melted (migmatitic) rocks (e.g., Redler et al., 2012, 2013; 55 Schmid and Wood, 1976; Zingg, 1980). This study focusses on metabasic rocks that are 56 interlayered with the metapelitic rocks in the Kinzigite Formation and which, though 57 volumetrically abundant, have received relatively little attention (Reinsch, 1973a,b; Rushmer, 58 1991; Sills and Tarney, 1984). Emphasis is on detailed field and petrographic observations 59 augmented with whole rock geochemical data that together provide evidence that, along with 60 the metapelitic rocks, the metabasic rocks within Val Strona di Omegna partially melted 61 during high temperature regional metamorphism. 62 63 2. Geological setting 64 65 The Ivrea Zone is a NW dipping and NE–SW striking slice of pre-Alpine metamorphic 66 basement located in northern Italy. It is bordered to the northwest by the Insubric Line (also 67 known as the Periadratic Line) that separates it from Alpine Units of the Canavese and Sesia 68 Zones (Gansser, 1968), (Fig. 1). The Insubric Line is a 1–2 km wide zone of intense 69 mylonitisation forming part of a major tectonic structure that can be traced from the French 70 Alps in the west to Greece in the east, and which separates the Central/Western Alps from the 71 Southern Alps (Gansser, 1968; Schmid et al., 1987). To the southeast, the Cossato-Mergozzo- 72 Brissago Line (CMB Line) and the younger Pogallo Line separate the Ivrea Zone from the 73 Strona-Ceneri Zone, which records greenschist to lower amphibolite facies assemblages and 74 contains granitic plutons of Permian age and coeval volcanic rocks (Quick et al., 2009) that 75 are locally covered by Mesozoic sediments. The Strona-Ceneri Zone represents a section 76 through a shallower crustal level to that exposed in the Ivrea Zone (Boriani and Sacchi, 1973; 77 Boriani et al., 1990), although it is unclear whether the Strona-Ceneri Zone and the Ivrea Zone 78 represent a once contiguous crustal fragment or are discrete crustal terranes that were 79 tectonically juxtaposed (Boriani and Sacchi, 1973). 80 In the southwestern part of the Ivrea Zone (Fig. 1), mafic rocks of the Mafic Complex 81 and ultramafic rocks of the Balmuccia mantle peridotite crop out close to the Insubric Line 82 (e.g., Quick et al., 1995, 2003; Rivalenti et al., 1975, 1981; Sinigoi et al., 1994, 2011). The 83 Mafic Complex, which reaches a maximum thickness of around 8 km in Val Sesia, is a 84 layered sequence of mafic/ultramafic rocks thought to have been formed by magmatic 85 underplating in an extensional environment (Quick et al., 1992). The Mafic Complex has been 86 subdivided into a lower unit, consisting mainly of amphibole gabbros and a ‘paragneiss- 87 bearing belt’ in which the mafic rocks contain septa of paragneiss, and an upper unit, which is 88 dominated by gabbros and diorites (Sinigoi et al., 1996). 89 The Kinzigite Formation crops out extensively within the central part of the Ivrea 90 Zone and is best exposed in Val Strona di Omegna (Fig. 1). It is cross cut by the Mafic 91 Complex in the southwest. The Kinzigite Formation comprises different rock types, the most 92 common of which are metapelitic and metabasic rocks with subordinate metapsammite, calc- 93 silicate, marble and metaperidotite. All rocks were regionally metamorphosed at amphibolite 94 to granulite facies conditions (e.g., Barboza and Bergantz, 2000; Barboza et al., 1999; Henk et 95 al., 1997; Peressini et al., 2007; Pin, 1990; Redler et al., 2012; Zingg, 1978) with regional 96 assemblages overprinted by contact metamorphism in close proximity to gabbroic rocks of the 97 Mafic Complex (e.g., Barboza and Bergantz, 2000; Barboza et al., 1999; Redler et al., 2012). 98 The pre-metamorphic history of the Kinzigite Formation is unclear, as the high-grade 99 metamorphism and deformation have erased almost all evidence for older events (e.g., 100 Schmid, 1993; Vavra et al., 1999). Peressini et al. (2007) date the intrusion of the Mafic 101 Complex to 288 ± 4 Ma and high-grade metamorphism in the Kinzigite Formation to 309 ± 3 102 Ma. More recent work by Ewing et al. (2013) suggests regional granulite facies 103 metamorphism in Val Strona di Omegna occurred at 316 ± 3 Ma. P–T estimates from the 104 Kinzigite Formation are mostly based on metapelitic samples and range from ~600°C and 3–4 105 kbar for the lowest grade rocks to in excess of 900°C and 10–12 kbar for the highest grade 106 granulite facies rocks (e.g., Bea and Montero, 1999; Ewing et al., 2013; Hunziker and Zingg, 107 1980; Luvizotto and Zack, 2009; Redler et al., 2012; Schmid et al., 1987). 108 A detailed petrographic study in Val Strona di Omegna (Reinsch, 1973a) subdivided 109 the metabasic rocks within the Kinzigite Formation (from relatively low to high grade) into: 110 (i) amphibolites dominated by hornblende and plagioclase with minor quartz and biotite; (ii) 111 garnetiferous metagabbros ('banded pyribolites') containing plagioclase, garnet, hornblende 112 and pyroxene and; iii) rare ‘pyriclasites’ containing plagioclase, clino- and orthopyroxene, 113 quartz and biotite. Based on their bulk rock major element composition, Reinsch (1973b) 114 classified the amphibolites as having alkali-basaltic to dacitic protoliths and the garnetiferous 115 metagabbro and 'pyriclasites' as representing metamorphosed alkali to hypersthene-bearing 116 basalts. P–T conditions were estimated at 700°C and 4–6 kbar for the amphibolite facies 117 metabasic rocks and 750–850°C and 6–8 kbar for the granulite facies metabasic rocks 118 (Reinsch, 1973b). Using trace element geochemistry, Sills and Tarney (1984) identified two 119 separate groups of amphibolites within Val Strona di Omegna. Type 1 amphibolites have trace 120 element patterns similar to N-MORB while type 2 amphibolites show patterns similar to E- 121 MORB.